Do NOT do as you have suggested and stick the roundels on by their perimeter only. Paul Madison did this and found to his cost that it caused blistering to the paintwork behind the unstuck sections of roundel that were holding moisture against the paint.

I'm hoping mine will come off ok later in the year, the paint was given several applications of wax before they went on. The roundels them selves are now pretty manky looking from all the numbers that have been on and off in the last couple of years.
I used the rectangles supplied by Merlin Motorsport, but if I was doing it again I would probably cut them down or buy suitable vinyl. The blue book essentially does specify the size as the numbers are supposed to be a certain height and there is a stated amount of border to these numbers which should be present, with a 3 digit number being the largest you would use.

But then you see people sticking hand drawn numbers on A4 paper in the back windows so....

J4 Page 147
Competition Numbers [Chart 4]4.1. Numbers must be displayed in a durable manner and be black, of a minimum size of 23cm high with stroke width of a minimum of 3.8cm on a white oblong background measuring not less than 48cm x 33cm which must extend at least 5cm beyond the outline of the numbers. Alternatively, the numbers may be displayed on a white circular background provided that the background extends at least 5cm beyond the outline of the numbers.
4.1.1. If on a white vehicle a background of the dimensions in 4.1. must be delineated by a continuous black line, except for cars of period A to E.4.1.2. Numbers must be displayed on each side of the vehicle (front doors, alongside the cockpit or on rear wing end plates) and on the foremost part of the nose.did one of the venues insist on the nose number one year?
4.1.3. Certain types of competition have special requirements which will appear in SRs (Subject to approval by the MSA) (see U.17.25 to 17.28 for Kart Numbers and Q11.4. for Circuit Racing).
4.1.4. Note. Consideration should be given at all times to the positive identification of vehicles by the competition organisers, whose decisions will be final.
4.1.5. The numbers must be of the ‘Classic’ type as shown below:
1–2–3–4–5–6–7–8–9–0.
4.1.6. In International events on both front wings an illustration of the national flag(s) of the driver(s) as well as the name(s) of the latter shall be displayed. Minimum height of both flag(s) and name(s) shall be 4cm.
4.1.7. Any means of identifying individual cars during a competition MUST be removed at the finish of the event.
4.1.8. Race organisers may use three figured numbers subject only to:
(a) the specific approval of the Chief Timekeeper
(b) that the white background respects the requirement of 4.1. above, which must extend at least 5cm beyond the outline of the numbers.

One other option might be the Frost peelable liquid wrap paint item code S700(ntdwm) - about a tenner for an aerosol of the stuff. Not used it myself and it would depend on number of coats needed and whether numbers would stick to it ok( or be able to be removed so different ones could be used).Might save door paint though.

1985 SS1 cvh. Galvanizing is for wusses with no sense of adventure.......